Fractus

Photo credit – cwPhotography – cc-by

Cloud of the Day – Fractus

Fractus are subsidiary clouds, also referred to as accessory clouds. This means that they are associated with primary clouds such as cumulus or stratus. That’s why fractus clouds are always named in combination with the cloud type that they are accessory to. When associated with cumulus they’re called cumulus fractus, with stratus, stratus fractus.

Photo credit – Andrew Basterfield – cc-by-sa

They’re called fractus because they have a ragged or shredded appearance. Cumulus fractus, for instance, looks like a nice, puffy cumulus cloud that has been torn up. Stratus fractus looks as if it’s been torn off the bottom of a layer of stratus cloud. Fractus clouds that form under the main cloud are often seen moving in a direction different from the overall movement. This can indicate what the wind is doing there and, if the speeds and directions differ significantly, can show instability and wind shear. This can be useful to pilots, who tend to call it scud cloud, for the way it scuds along. There’s also an acronym: scattered cloud under deck.

Photo credit – Dominiktesla – cc-by-sa

Fractus clouds are good weather indicators. They often form under rain clouds, including cumulonimbus, and at the leading and trailing edges of weather fronts. If the stormy weather hasn’t struck yet, fractus can be a sign that it soon might.

Photo credit – Merikanto – cc-by-sa

No precipitation comes from the fractus itself, but may come from its parent cloud.

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rjb